1. Field of the Invention
This invention relates to a semiconductor laser, and more particularly to a composition of a semiconductor layer for forming a index optical waveguide semiconductor laser.
2. Description of the Related Art
Recently semiconductor lasers have come to be wide used in light communication, optical discs, laser printers and the like. Advent of high output semiconductor lasers not lower than 1W has realized application of semiconductor lasers to sublimation of dye by use of a laser spot as a high density heat source or to image forming, processing of material or soldering by use of laser ablation.
There have been made various attempts to make more stable a semiconductor laser and to increase output power of a semiconductor laser. For instance, as disclosed in "Applied Physics Letters", Vol. 69(1996) pp. 1532 to 1534, there has been proposed a 980 nm band semiconductor laser comprising an n-InGaP clad layer, an InGaAsP optical waveguide layer, an InGaAs strained double quantum-well active layer, an InGaAsP optical waveguide layer, a p-InGaP clad layer, and a p-GaAs cap layer formed on an n-GaAs substrate. In this semiconductor laser, the thicknesses of the optical waveguide layers are increased to reduce internal loss and to reduce the density of light in the active layer, thereby obtaining a light output of 8W for a stripe width of 100 .mu.m.
As a semiconductor laser in which the output power is increased by increasing thickness of the optical waveguide layer, there have been reported in "SPIE Proc., Vol. 3001(1997), pp. 7-12" a semiconductor laser having an active layer formed of InGaAsP and an optical waveguide layer and a clad layer formed of InGaAlP and a 810 nm band semiconductor laser which comprises an InGaAsP active layer, an InGaP optical waveguide layer, an n-In.sub.0.5 Ga.sub.0.25 Al.sub.0.25 P clad layer, a p-In.sub.0.5 Ga.sub.0.25 Al.sub.0.25 P first upper clad layer, a p-In.sub.0.5 Ga.sub.0.45 Al.sub.0.05 P second upper clad layer, and a p-AlGaAs carrier blocking layer disposed in a boundary region of the first and second upper clad layers.
In a semiconductor laser, an index optical waveguide structure such as a ridge structure is generally formed in order to control the fundamental transverse mode. The index optical waveguide structure is formed by etching the epitaxial layer on each side of the ridge stripe up to a certain depth of the upper clad layer or the optical waveguide layer with the upper clad layer or the optical waveguide layer left there in a certain thickness. Since the oscillation mode is governed by the thickness of the upper clad layer or the optical waveguide layer left there, the depth of etching must be carefully controlled. However in the high output semiconductor lasers described above, it is very difficult to stop etching at a desired depth in forming the index optical waveguide structure due to their arrangement of the semiconductor layers. Accordingly, it is difficult to manufacture a high output semiconductor laser oscillating in a controlled transverse mode at a high repeatability.